Apparatus for making prestressed concrete members

ABSTRACT

AN APPARATUS FOR FORMING PRESTRESSED CONCRETE PRODUCTS. A STRESS FRAME INCLUDING PROPERLY TENSIONED TENDONS IS LOWERED OVER A CASTING FORM WHICH DOES NOT FEEL THE TENSION. THE CONCRETE IS POURED, COMPACTED, AND ALLOWED TO REACH A PRE-SET CONDITION IN WHICH IS IS SELF-SUPPORTING YET PLASTIC. THE STESS FRAME, WITH THE PRODUCT ATTACHED THERETO, IS THEN THE CASTING FORM, AND THE PRODUCT UNDERGOES FINAL CURING WITH ALL ITS EXTERIOR SURFACES EXPOSED. THE TENDONS ARE THEN RELEASED, TRANSFERRING TENSION TO THE CONCRETE PRODUCT.

May 30, 1972 R. s. BAKER 3,666,335

lzll'laf ifl'l'flfi FOR MAKING PRESTRESSED CONCRETE MEMBERS Filed July 5, 1969 4 Sheets-Sheet 1 H 1 INVENTOR.

ROBERT S. BAKER BY wwwmw ATTORNEYS F2. 5. BAKER 3,666,385

CONCRETE MEMBERS May 30, 1972 APPARATUS FOR MAKING PRESTRESSED 2 4 Sheets-Sheet 2 Filed July 5. 1969 i all NIH C IJIW IIH' I INVENTOR ROBERT S. BAKER BY WM ATTORNEYS R. s. BAKER May 30, 1972 APPARATUS FOR MAKING PRESTRESSED CONCRETE MEMBERS 4 Sheets-Sheet :5

Filed July 5, 1969 INVENTOR ROBERT S, BAKER ATTORNEYS May 30, 1972 R. s. BAKER 3,666,385

APPARATUS FOR MAKING PRESTRESSED CONCRETE MEMBERS Filed July 3, i969 Q 4 Sheets-Sheet 4 I-NVENTOR. ROBERT; s'. BAKER ATTORNEYS United States Patent 3,666,385 APPARATUS FOR MAKING PRESTRESSED CONCRETE MEMBERS Robert S. Baker, 511 Hibiscus Drive, Temple Terrace, Fla. 33617 Filed July 3, 1969, Ser. No. 838,945 Int. Cl. B28b 7/08 US. Cl. 425-111 2 Claims ABSTRACT OF THE DISCLOSURE An apparatus for forming prestressed concrete prod ucts. A stress frame including properly tensioned tendons is lowered over a casting form which does not feel the tension. The concrete is poured, compacted, and allowed to reach a pre-set condition in which it is self-supporting yet plastic. The stress frame, with the product attached thereto, is then separated from the casting form, and the product undergoes final curing with all its exterior surfaces exposed. The tendons are then released, transferring tension to the concrete product.

The present invention pertains to prestressed concrete. More particularly, the present invention pertains to an improved apparatus for making prestressed concrete objects.

Prestressed concrete is utilized in many different manners in the construction industry, not only in structures such as buildings and poles, but also in slabs such as roadway pavement. Prestressed concrete offers advantages in construction methods and in quality of the finished products. Heretofore, prestressed concrete objects have been made in one of two basic manners. In the first of these, abutments are secured to the earth and prestressing tendons are stretched between the abutments to a predetermined tension. Casting forms for the desired product lie between the abutments but do not feel the tensioning forces, these being borne by the abutments. The concrete is then poured into the casting form, consolidated, and allowed to remain in place in the casting form until the concrete has reached sufificient strength to permit transfer of the prestress forces to the concrete. The tensioning forces are then released from the abutment, and the tendons attempt to return to their original length and diameter. The mechanical bond between the concrete and the tendons prevents this return, and the resulting forces create compression stresses in the concrete. After this transfer of forces, the product is removed from the casting form.

The second method is similar to the first except that no abutments are used. The casting forms are so constructed that they are capable of withstanding the pretension forces, and the skin of the casting forms acts as a column to hold the forces until such time as the concrete has reached sufficient strength to permit the force transfer to the product. Only then can the forces be released to the product and the product removed from the form, as in the first method.

Both of these known methods of producing prestressed concrete objects have several drawbacks. The product must remain in the casting form for the complete curing time, which generally is in the order of a full day. Consequently, the casting form can be utilized for only one production cycle per working day. The product cannot be inspected prior to its removal from the casting form, by which time hydration is well advanced, and the concrete has lost all plasticity. Elastic shortening of the product occurs when the prestress forces are transferred to the product. This causes movement of the product relative to the casting form surfaces and results in hazards of spalling and breakage of the product. Since most or all of the surface of the product is covered by the casting form, that surface is not easily exposed to the desirable temperature and humidity conditions during final curing.

The present invention is an improved apparatus and method for forming prestressed concrete products. In accordance with the present invention, prestressing tendons are stretched across a stress frame comprising a plurality of columns capable of withstanding the pretension forces. After the tendons are properly tensioned, the stress frame is placed on a casting form module which is completely independent of the stress frame and thus feels none of the forces in the tendons. The concrete is then cast, compacted, and allowed to attain its preset condition in which it is self-supporting. The stress frame, with the preset concrete product on the tendons is then separated from the casting form, with the prestress tension forces still borne by the stress frame. As a result, all of the product surface is exposed during the curing of the concrete. In addition, the casting form is available for reuse. The stress frame with the preset product is then moved to any desired location which might be the final site or a curing oven with controlled temperature and humidity. After the concrete has cured, the tendons are released from the stress frame, transferring the prestress forces to the concrete product. Since the product is not in contact with any casting form at the time the prestress forces are transferred, there is no hazard of spalling or breakage of the product. Once the tendons are released, the stress frame is available for reuse, and the product, now prestressed, is left in proper orientation and position for use or storage.

The present invention thus provides a prestressed concrete product which is separated from its casting form prior to the concrete reaching transfer strength and prior to release of the pretensioned tendons. The invention permits reuse of the casting form during a single production cycle and eliminates the hazards of shrinkage and elastic shortening by separating the product from its casting form prior to detensioning of the tendons. The product can be inspected while the concrete is still in the plastic state, and so, if needed, minor surface repairs can be easily made. Since the product is out of the casting form during curing, all of the product surface is exposed to the humidity and temperature conditions which are conducive to rapid and uniform curing. By use of multiple casting forms, in accordance with this invention, only one-time handling of multiple units is required, and that handling is prior to transfer of the prestress forces to the product...

These and other aspects and advantages of the present invention are apparent in the following detailed description and claims, particularly when read in conjunction with the accompanying drawings in which like parts bear like reference numerals. In the drawings:

FIG. 1 is a side elevational view of one embodiment of a multiple casting apparatus for simultaneously making a plurality of prestressed concrete products in accordance with the present invention;

FIG. 2 is a plan view of the apparatus of FIG. 1;

FIG. 3 is an end elevation view of the apparatus of FIG. 1 and depicting the stress frame in a raised position;

FIG. 4 is an enlarged, vertical sectional view of the apparatus of FIG. 1 and depicting the stress frame with the preset product in a raised position and is taken along line 4-4 of FIG. 5; and

FIG. 5 is an enlarged, fragmentary plan view of the forming assembly utilized with the apparatus of FIG. 1.

FIGS. 1, 2 and 3 depict an assembly 10 capable of simultaneously forming up to four prestressed concrete pieces. Assembly 10 includes a movable crane assembly 12, such as a roto-crane, and a forming assembly 14. Forming assembly 14 includes a plurality of casting forms 16 and a stress frame 18. Stress frame 18 is connected by trunnions 20 to screws 22 of crane 12. As seen in FIG. 4, each casting form 16 comprises essentially a female mold in the shape of the desired 'product. Stress frame 18 is designed to match the casting forms 16 and can accommodate a plurality of casting molds, as depicted. Stress frame 18 includes a plurality of tension resisting support columns 24. As depicted in the illustrative example of FIG. 4, each support column 24 includes a pair of tubes 26, of for example steel, and a channel 28, which likewise can be of steel. Each end of support column 24 abuts a rigid frame end member such as header plate 30, as shown in FIG. 5. Columns 24 are positioned to lie on each side of each casting form 16 when stress frame 18 is lowered onto the casting frames. Trunnions 20 are connected to the outer side of each header plate 30.

Between each adjacent pair of support columns 2 4 a plurality of tension means such as cable tendons are stretched from one header plate 30 to the other. Thus, as depicted in FIGS. 4 and 5, between each pair of columns 24 four tendons 32 are stretched between the two header plates 30. Suitable means such as strand chucks 34 are provided on each end of each tendon 32 to secure the tendons to header plates 30. At each end of each casting form 16, an end plate 36 is provided and, for example, might be welded in place. End plates 36 include slots 37 to permit tendons 32 to be inserted thereinto while remaining substantially mechanically independent therefrom as stress frame 18 is lowered onto casting forms 16. The interior surface of casting forms 16 is coated with a suitable material such as Teflon to prevent concrete from adhering thereto.

When prestressed concrete products are to be made, casting forms 16 are empty and clean. Then tendons 32 within stress frame 18 are properly tensioned, for example, by means of a stress jack such as the jack depicted in U.S. Pat. No. 3,176,962, issued Apr. 6, 1965 to Robert S. Baker. Tendons 3-2 are retained in this tensioned condition by strand chucks 34. Stress frame 18 is then lowered into position over casting forms 16, as depicted in FIG. 2. Since casting forms 16 are substantially mechanically independent of stress frame 18, none of the tension of tendons 32 is felt by the casting forms. The slots 37 in end plates 36 are closed by tape or other suitable means. Each casting form 16 is filled with pourable concrete in a slurry form which is then compacted, for example by an internal or an external vibrator. The concrete is then allowed to remain in place within the casting forms until it attains a preset condition in which the concrete is self-supporting but plastic. The length of time required for the concrete to reach this preset condition will depend upon the cement used and upon ambient conditions and might be in range of about one and one-half to about four hours and typically about two hours.

After the concrete has reached the preset condition, crane 12 is connected to trunnions 20 to raise stress frame 18 with the preset concrete products, as depicted in FIGS. 3 and 4. Crane 12 moves along tracks 38, carrying stress frame :18 and each product 40 and leaving casting forms 16 in place and available for reuse. The tensioning forces are still borne by stress frame 18.

Stress frame 18 is rotated 180 about trunnions 20 to orient product 40 face up. Crane 12 then moves stress frame 18 and product 40 to the desired location for final curing, which may be the location of use or may be a storage location. While the product 40 is undergoing final curing, stress frame 18 can be removed from crane 12 so that the crane is not incapacitated during this time. The entire exterior surface of each product 40 is exposed,

which results in final curing of product 40 taking place in the desired ambient conditions. In addition, the product can be visually inspected while still in a plastic state, and any necessary minor surface repairs can be made prior to hardening. After the product 40 has cured to transfer strength in which it can withstand the tension of tendons 32, typically another 8 to 12 hours, the closure is removed from slots 37, and tendons 32 are released from stress frame 18, either by releasing strand chucks 34 or by cutting each tendon 32 adjacent end plates 36. This transfers the prestress tension forces to product 40. Since product 40 is not in contact with casting forms 16, this tension force transfer takes place without damage to product 40. Stress frame 18 is then available for reuse.

I claim:

1. Apparatus for making prestressed concrete objects comprising:

(I) a stress frame including:

(a) a first rigid frame end member having a first trunnion extending from one side thereof;

(b) a second rigid frame end member having a second trunnion extending from one side thereof;

(c) a plurality of elongated rigid support members rigidly connecting said first rigid frame end member and said second rigid frame end member;

(d) a plurality of cable tendons extending between said first rigid frame end member and said second rigid frame end member; and

(e) means for maintaining said cable tendons in a stressed condition between said first and second rigid frame end members; and

(II) a casting form means substantially mechanically independent from the stress frame, including a mold of the prestressed concrete object having a bottom member, first and second side members, and first and second end members, each of said end members having at least one slot therein to permit placement of said stress frame about said casting form means with said cable tendons within said mold while remaining substantially mechanically independent and removable therefrom;

whereby by lifting said trunnions a cast concrete object having reached a self-supporting, preset condition can be removed from said mold to expose all surfaces thereof during final curing while keeping tension therefrom during such time.

2. Apparatus as claimed in claim 1 further comprising a crane mounted on rails for movement thereon and adapted for attachment to said first and second trunnions to permit movement of the rigid, stressresisting frame.

References Cited UNITED STATES PATENTS 2,397,728 4/1946 Dowett et al. 25-118 T 2,556,928 6/ 1951 Ludlon 2 51 18 T 2,590,478 3/1952 Weinberg 25-119 X 3,118,211 1/1964 Baber et a1 251 18 T 3,128,521 4/1964 Baker 25--1 18 TX 3,233,027 2/1966 Wennstrdm 25--=l18 TX I. SPENCER OVERHOLSER, Primary Examiner B. D. TOBOR, Assistant Examiner U.S. Cl. X.R. 249-86, 94 

